It is known that certain mineral matter, especially perlite or vermiculite, can be expanded or blown by subjecting the particles thereof to elevated temperatures.
Apparatus for this purpose can comprise a fluidized bed shaft-type furnace, at least one burner for introducing a fuel and combustion sustaining air into the fluidized bed furnace, and means for introducing particles of the mineral matter to be blown into the fluidized bed whereby the interaction between the heat in the fluidized bed and the particles causes the particles, while they are circulating in the fluidized bed, to expand.
Generally, the burner opens into a lower portion of the fluidized bed shaft while the particle-feed device introduces the particles into the fluidized bed at a location well above the burner, e.g. from a feed tube or the like.
The fluidized bed may be of the so-called expanded type whereby the gas stream, emerging from the top of the bed, entrains the expanded perlite or vermiculite particles from the bed or shaft.
Downstream of the shaft, these particles can be separated from the gas stream and collected.
For the most part, the burner also serves to supply to the furnace, the fluidizing air or gas which generates the fluidized bed movement within the shaft and upwardly along the latter to the outlet.
The shaft can have a height of several meters and the particles are generally fed into the shaft in the lower half of the bed.
Initially, these comparatively dense particles engage in a free fall in which they move counter to rising currents of hot gas above the flame. Ultimately they are circulated into the region of the flame and expanded. Prior to expansion and the desired degree of contact with the flame, however, they may be entrained upwardly, turned, fall downwardly, turned, are carried upwardly a number of times. The interaction is thus a function of the aerodynamic characteristics and statistical variations.
Experience has shown that, for a given output or throughput of the mineral matter, considerable energy is wasted, presumably because of ineffective movement of the particles between the time they are introduced and the time they interact with the flame or in the bed. Furthermore, because the interaction depends in large matter on statistical circumstances and aerodynamic factors, the product which is obtained is frequently nonuniform. This may be a result of the repeated subjection of the particles to severe thermal gradients, in part the result of widely varying residence times in the fluidized bed and like differences. The product, therefore, is generally considered to have poor quality since the apparent density of the product and the pore volume or porosity may vary. The problem is especially acute because a sharp temperature rise, which is necessary for expansion of vermiculite or perlite, cannot always be assured for all particles.
In one earlier system (German open application No. 20 42 896) by a corresponding feed of combustion air in the burner, a turbulent flow of the combustion gases can be generated to provide, in the downwardly conical tapering fluidized bed furnace, a condition in which the mineral matter rides along the wall of the furnace until it reaches the conical region where the particles are picked up by the turbulent flow.
The use of a turbulent stream in this fashion is intended to bring about a more uniform treatment of the mineral matter, but here also individual particles may be subjected to sharply different heating patterns and thus give rise to blown products of varying quality.
In this earlier system, the gases and particles move especially in the coincal region as in a cyclone so that the heavier particles are thrown outwardly and thus are subjected to a different treatment from the lighter particles.
Similar problems with the system are described in U.S. Pat. No. 2,435,927 in which, moreover, the combustion gas and particles are introduced together into the cylindrical portion in a parallel flow, only thereafter entering the turbulent flow.